Abstract

The spiny-cheek crayfish, Orconectes limosus (Rafinesque, 1817) and the signal crayfish, Pacifastacus leniusculus (Dana, 1852) are fast spreading non-indigenous invasive North American crayfish species colonizing the European freshwater ecosystems. Both invasive species are the crayfish plague carriers and represent a serious threat to the native crayfish fauna. The non-indigenous invasive spiny-cheek crayfish has spread considerably throughout the Danube and the main tributaries. Through the deliberate introduction it occurs in the Tisa River (middle section) and in Sebes-Körös River in Hungary. The signal crayfish is known to be distributed in the upper section of the Danube River (Austria) extending its range down the river to its middle section. Currently, the signal crayfish occur in the vicinity of Bratislava (Slovakia) as well. The species is recorded in the Drava Basin (mainly in the Austrian tributaries of the Drava River) but recently, through the deliberate introduction, occurs in the Sava Basin (Danube tributary) also. Based on estimated colonisation rates we evaluated that in the next decades, both species will with high probability double their range of occurrence in the Danube Basin, accompanied with considerable negative impact not only on the native crayfish assemblages but the whole aquatic ecosystem. This paper summarizes all published data considering records of O. limosus and P. leniusculus in Central and South-Eastern Europe related to the Danube Basin and evaluates their dispersal trends in the region.

Freshwater ecosystems are one of the major components of the Earth's nature. Even though fresh water occupies only 0.8% of the Earth's surface and represents only 0.01% of all water on Earth, it is still a major natural element providing habitat for more than 100,000 species (Dudgeon et al., 2006). Due to overexploitation, water pollution, flow modification, destruction of habitat and invasion of exotic species, and also considering the effects of climate change, this natural element is seriously endangered (Dungeon et al., 2006; Gallardo and Aldridge, 2013; Capinha et al., 2013). European freshwater crayfish species are among the taxa that has undergone major turbulences due to water quality modifications and pollution and extreme overexploitation for commercial purposes (Souty-Grosset et al., 2006). Recently, one of the main causes of native crayfish populations decline is the introduction of the non-indigenous invasive crayfish species - the crayfish plague carriers (Holdich et al., 2009). Although these introductions started at the end of the 19th century, this issue of introduction continues, transforming the subject to a continental problem in Europe with native population declines at all sites of exotic species occurrence (Schultz et al., 2006). Several non-indigenous crayfish species were introduced in Europe primarily for commercial purposes (food and aquaristic) (Souty-Grosset et al., 2006; Holdich et al., 2009). Introductions of the non-native species progressed through the 20th century on, introducing the non-indigenous species intensively from one water body to another (Souty-Grosset et al., 2006). The competition for shelter and food in native - non-native crayfish interactions promoted the decrease of the native crayfish populations considerably (Stebbing et al., 2006; Bubb et al., 2006). This has negatively affected the native populations fostering their decrease of already overexploited populations, as in the case of the native European crayfish (noble crayfish or broad-fingered crayfish) Astacus astacus (Linnaeus, 1758).

The introduction of Orconectes limosus (Rafinesque, 1817) started in Central Europe. The species was first introduced to a fish pond near a tributary (Mysla River) of the Oder River at the end of 19th century (1894) in Germany (Smolian, 1926). A second release of the species was carried out in a tributary of the Loire River in 1912, France (Pretzmann, 1994). This species is extensively distributed in North-Western Poland mainly along the Oder River. The species reached North-Eastern Germany where it expanded to every water body in the Mecklenburg-Vorpommern region (Zettler, 1998). O. limosus occurs in the upper part of the Vistula River (Poland) but a colony has also been detected in the middle part of the river (Schulz and Smietana, 2001; Grabowski et al., 2005).

The species is recorded in the North-Eastern part of Poland, reaching Kaliningradskaya (Russia) and in North-Western Belarus and South-Eastern Lithuania (Burba, 2010). Germany was the first country to have O. limosus in natural ecosystems, but the species was not recorded until 1947 when it was found in the Main River near Frankfurt (Geelen, 1978). The species expanded drastically in Germany and now occurs in river systems like: Necker, Weser, Mosel, Nahe, Main, Aller, Ems, Elbe, Rhine, Lahn, Midland Canal, Lippe and many others (Geelen, 1978). The distribution of the species in the Czech Republic was recently evaluated in detail by Petrusek et al. (2006). The species could have been present in the territory of the Czech Republic already in the 1960s but the first data were published only much later, in 1989 (Hajer, 1989). Current occurrence of the species in the Czech Republic includes rivers as: Ohre, Vltava, Jizera, Mrlina, Cidlina, Doubrava, Metuje and Upava (tributaries to Elbe) and Otava, Luznice, Sazava and Malse (triburaties to the Vltava) (Petrusek et al., 2006). The core of O. limosus distribution remains in the Elbe River, where the species occurs throughout its whole navigable part (Petrusek et al., 2006).

Pacifastacus leniusculus (Dana, 1852) was first introduced to Europe in 1959 to Sweden in attempt to substitute the declining A. astacus populations (Souty-Grosset et al., 2006). The species is common mainly in Northern Sweden, Denmark and Finland (Westman, 1995; Edsman, 2004; Skov et al., 2011; Vralstad et al., 2011) and Western Belgium, France, Spain and England (Holdich, 2002; Holdich et al., 2009). There are also some populations in Northern Poland, Lithuania, Latvia and Estonia (Souty-Grosset et al., 2006). P. leniusculus occurs also in Greece, where it was deliberately introduced in 1982 (Holdich et al., 2009).

Native crayfish species are nowadays endangered on a continental scale due to rapid colonisation by non-indigenous invasive crayfish species. Thus, orientation towards their protection has become a must. South-Western Romanian populations of the stone crayfish Austropotamobius torrentium (Schrank, 1803) are recently classified as endangered due to further colonization of the exotic crayfish species O. limosus both from Hungary (Tisa River) (Pârvulescu and Zaharia, 2013) and Serbia (Tamiš River) (Lipták et al.). A. astacus and Austropotamobius pallipes (Lereboullet 1858) are listed as critically endangered and A. torrentium as endangered in Germany as well (Chucholl, 2013). The decrease of native species is also recorded in Austria and Hungary (Wainländer and Füreder, 2009; Györe et al., 2013). The situation in other countries of the Danube Basin is no different and the conservation of native crayfish species is due to these facts inevitable. This paper presents an overview on the current distribution of the two non-indigenous invasive crayfish species with insight into possible dispersal trends and evaluates possible future distribution of the two species in the Danube Basin. This should point out to the importance of the species colonization potential and alarm for the need for conservation practices of the native crayfish species that are due to above mentioned causes seriously threatened.

Orconectes limosus

Introduction of O. limosus in the Danube Basin started in 1959 in Hungary, where several thousand individuals were released in vicinity of Budapest (Thuránszky and Forró, 1987). The first records of O. limosus in the Danube (1985) were confirmed from Hungary and Germany (Bavaria), where the species were found in the vicinity of Budapest (Hungary) and Ingolstadt (Germany) (Thuranszky and Forro, 1987; Nesemann et al., 1995). O. limosus spread fast along the Danube River in Hungary, colonizing the whole Hungarian stretch of the river by now (Puky and Schad, 2006). Approximately at the same time as the species reached Serbia, its occurrence was reported from the middle section of the Tisa River in Hungary in 2005 (deliberate introduction). O. limosus is present in several tributaries of the middle Tisa River as well, colonizing new areas fast (Sallai and Puky, 2008; Szepesi and Harka, 2011; Györe et al., 2013). This species was not recorded in Slovakia until 2007, when the species was detected in the lower stretch of Ipeľ and Váh - major tributaries of the Danube in Slovakia (Janský and Kautman, 2007). The species was confirmed for the Danube River in Slovakia later in 2008, near the border with Hungary (Puky, 2009). The O. limosus occurrence was confirmed on several sites along the Danube in 2013, near the villages Patince and Zlatná na Ostrove (B. Vitázková, personal data). At present, the spiny-cheek crayfish occurs in the vicinity of Bratislava as well (Lipták, 2013). The occurrence of the species in the vicinity of Bratislava is most probably a result of persisting dispersion of O. limosus (Hungarian population) in the upstream of the river Danube. However, there is a second population occurring in Austria (Danube) and the Morava River, with unknown population status. A sustaining population is identified in the region of Vienna, discovered in 1991, and in the Morava River near Schlosshof in 1990 (although its occurrence in the Morava River has not been confirmed since) (Nesemann et al., 1995; Pöckl and Pekny, 2002).

South-Western Slovakia could have been inhabited by O. limosus back in the 1990s, but remained undetected until recently, remaining most probably at low densities. This invasive species was found in Serbia in 2002 in the Danube River near Apatin and later in 2004 in the vicinity of Smederevo (Karaman and Machino, 2004; Pavlović et al., 2006). The spiny-cheek crayfish was detected in 2003 in the Park of Nature Kopački rit in Croatia in the flood zone bordered by the Danube and the Drava Rivers (Maguire and Klobučar, 2003; Maguire and Gottstein-Matočec, 2004;), and spread considerably in the region of its first observation, with progressive colonisation toward the Drava River in Croatia (Hudina et al., 2009; Maguire et al., 2011). The species was recorded in the Tamiš River (Serbia) in 2011, indicating its expansion trend to Danube River tributaries in the region (Lipták et al., 2013). In 2008, O. limosus reached the Romanian Danube (Parvulescu et al., 2009). O. limosus spread successfully in the Balkan region, thus the occurrence of the species can be even broader than documented so far (Lipták et al., 2013).

Estimated colonization speed of the spiny-cheek crayfish in the Danube is set within a range of 13-16 km yr-1 (Puky and Schád, 2006; Pârvulescu et al., 2012). In the Drava River the colonization speed was calculated to be 2.5 km yr-1 (Hudina et al., 2009). Furthermore, the data obtained by Hudina et al. (2009) indicate an average colonization speed in the Danube River to an incredible 84 km yr-1 in Croatia and Serbia, 48 km yr-1 in Romania and 13 to 16 km yr-1 in Hungary (Puky and Schád, 2006). Through these estimations and the evaluated colonisation trends, the future distribution of O. limosus can be considerable (Figure 1, Figure 2).

Figure 1: The distribution and colonization pattern of O. limosus in the South-Eastern range of its occurrence in Europe. The solid lines represent the confirmed direction of colonization. The dashed lines represent a potential high-likely direction of colonization. The black diamonds represent isolated introduced populations.

Figure 2: The distribution and colonization pattern of O. limosus in Central Europe. The solid lines represent the confirmed direction of colonization. The dashed lines represent a potential high-likely direction of colonization.

Pacifastacus leniusculus

P. leniusculus was introduced in the 1970s to Austria and to Czech Republic in the 1980s for commercial purposes (Holdich et al., 2009). The species has spread especially due to human mediated introductions to many water bodies, from where it has later spread actively, colonizing new areas outside its breeding sites into the wild. In Central Europe its main abundance rests in Austria (Pöckl, 1999). P. leniusculus later spread to Slovakia invading the Morava River (although human mediated transport cannot be excluded here as well) (Petrusek and Petrusková, 2007). P. leniusculus was also recently discovered in the Danube River near Bratislava (Lipták, 2013), suggesting an extending trend of the Moravian population. The first record of P. leniusculus in the Danube Basin dates back to 1985, when the species was introduced to Carinthia (Austria) (Weinländer and Füreder, 2009). The species was introduced to Isel, Drava, Glan, Gurk and Lavant Rivers (Danube Basin rivers in Austria), from where the species has spread intensively (Wainländer and Füreder, 2009), causing dramatic decline of the local populations native crayfish.

In 2008, the signal crayfish reached Croatia, spreading through the Mura River, colonizing the whole stretch of the Mura in the country (Maguire et al., 2008). Later, the species reached Slovenia spreading from the Drava River (from Austria) (Maguire et al., 2011). The species was recently discovered in the Korana River (Sava Basin) in Croatia as well, where the species was introduced through human mediated transport (Hudina et al., 2013).

The dispersal trends of the signal crayfish are estimated in Figure 3. The colonization speed of the signal crayfish was previously calculated to be in a range of 18-24 km yr-1 (Hudina et al., 2009) and 1.4 km yr-1 for Korana River (Hudina et al., 2013). By these estimations it is clear that the future distribution can broader considerably in comparison to current occurrence in just few decades.

Figure 3: The distribution and colonization pattern of P. leniusculus in range of its occurrence in the Danube Basin in Europe. The solid lines represent the confirmed direction of colonization. The dashed lines represent a potential high-likely direction of colonization. The black diamonds represent isolated introduced populations.